Epoxyaurapten inhibition of smooth muscle contraction and phosphorylation of myosin light chain by myosin light chain kinase

Epoxyaurapten is a component of coumarin, which was isolated from Aurantii Fructus Immaturus. The objective of this study was to determine the inhibition of epoxyaurapten on smooth muscle in vitro. The experiment of smooth muscle contraction directly monitored the contractions of the isolated proximal duodenum by frequency and amplitude at different epoxyaurapten concentrations and under different incubation times. The results showed that epoxyaurapten inhibited contraction in intestinal muscles in a dose-and time-dependent manner. The effects of epoxyaurapten on myosin were measured in the presence of Ca2+-calmodulin using the activities of 20 kDa myosin light chain (MLC20) phosphorylation and myosin Mg2+-ATPase. The results demonstrated that MLC20 phosphorylation by myosin light chain kinase (MLCK) decreased with increasing epoxyaurapten concentration. The myosin Mg2+-ATPase activity also gradually deceased with increasing epoxyaurapten concentration. With prolonged incubation time, MLC20 phosphorylation by MLCK and myosin Mg2+-ATPase activities furthered declined. The effect of epoxyaurapten on MLCK expression was measured by western blot, and the results showed that epoxyaurapten inhibited the expression of MLCK in a dose-and time-dependent manner. The findings demonstrated that epoxyaurapten could effectively attenuate the contractions of intestinal smooth muscle and therefore could be developed as a potential therapy in the future.